1. Field of the Invention
This invention relates to the manufacture of integrated circuits and, more particularly, to selective oxidation of a gate conductor having a layer of refractory metal and a layer of polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d). Selective oxidation is achieved by using an improved atmospheric pressure furnace that minimizes oxidation of the refractory metal sidewall yet allows oxidation of the polysilicon sidewall by flowing nitrogen at critical areas of the furnace to minimize oxygen leakage into the furnace while the furnace is closed and an oxide reducing gas, such as hydrogen, is not present.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
The manufacture of an integrated circuit involves many steps. For example, a gate conductor is formed by selectively removing polysilicon that had been previously blanket deposited across a semiconductor topography. The remaining polysilicon can thereafter be used as a mask when forming the junction regions into the underlying substrate. A polysilicon gate is therefore aligned between the underlying junction, and is sometimes referred to as a xe2x80x9cself-aligned gate.xe2x80x9d
Many modem gate structures often involve multiple layers besides simply a polysilicon layer dielectrically spaced above a channel region. For example, the gate conductor can have a refractory metal layer aligned above the patterned polysilicon. The refractory metal can serve as a low resistive strap that essentially shorts the junction between the p-type and n-type polysilicon. Instead of placing the refractory metal only in the strap areas applicable to a complimentary metal-oxide-semiconductor (CMOS) device, the refractory metal can extend along the entire polysilicon trace, if needed, to help reduce the effective sheet resistance of the polysilicon gate and improve adhesion of the polysilicon to an overlying metal trace conductor that might extend across a majority of the integrated circuit.
A problem attributed to a stacked polysilicon and refractory metal gate conductor is that each layer will react differently when exposed to an oxidation step. For example, it may be necessary to oxidize the sidewall surface of the gate conductor in order to form a graded junction, using the well-known lightly-doped drain (LDD) technique. The oxidation step not only forms spacers on the sidewall surfaces of the gate conductor, but also helps heal any damage done to the polysilicon as a result of the previous etch step. The healing process is often accomplished by an anneal step. In order to anneal whatever damage might exist in the polysilicon, the gate conductor must be subjected to a relatively high temperature to convert the amorphous silicon into a more stable polysilicon composition.
Whenever the exposed sidewall surface of the polysilicon gate is subjected to oxygen at a high temperature, an oxide will grow on that polysilicon, possibly, in the form of a spacer. The same can be said of any overlying refractory metal sidewall surface that is also exposed to the oxidizing ambient. Depending on the refractory metal and the conditions in which the refractory metal is exposed to oxygen at a high temperature, the refractory metal may be entirely consumed or only partially consumed by the oxygen. In a worst-case scenario, the refractory metal might become entirely consumed and the structure of the refractory metal jeopardized so that any overlying capping layer of the gate conductor will dislodge from the gate conductor itself. In a less than worst-case scenario, the refractory metal is only partially consumed at the sidewall surface and the gate conductor presents a hillock, bump, or whisker that extends laterally outward from the gate conductor.
While it is beneficial to oxidize the sidewall surface of a gate conductor, it is far more important to only selectively oxidize that surface. In other words, it would be desirable to introduce a process that selectively oxidizes only the polysilicon sidewall surface, but not the refractory metal sidewall surface. An improvement is therefore needed in the oxidation process that would essentially minimize the introduction of oxygen during critical heating of the oxidation tube to effectuate the oxidation process. Many conventional methodologies teach the use of specially designed low pressure furnaces or tubes that carry out the oxidation process, or tubes that remain relatively cool until after wafers are pushed into the tube and thereafter the tube is heated to an oxidation temperature. While it is relatively known that refractory metal does not oxidize at lower temperatures, the concept of using low pressure furnaces that are relatively cool and then having to ramp up the temperature to achieve an oxidation temperature significantly lessens the throughput of the oxidation process. Therefore, selective oxidation of polysilicon and not refractory metal must be done in a way that whiskers do not form and throughput does not suffer.
The problems outlined above are in large part solved by an improved furnace and a process methodology that uses the improved furnace. The improved furnace is one that includes a tube that is preferably preheated to a relatively high temperature. Aligned with the tube is a plurality of wafers placed within a boat, and the wafer-containing boat is maintained in a load-lock area. The load-lock area or boat handing area contains essentially only an inert ambient, such as nitrogen gas. The boat is then inserted into the preheated tube, which is then sealed.
Even though the tube is sealed after it is loaded, there may be inadvertent leaks that form in or around the doorway into the tube, as well as in the inlet or exhaust lines to and from the tube. Placed around each of those potential leak areas is preferably a container. The container can be configured as a box, for example, having an opening that is secured preferably to an exterior surface of the tube around the potential leak area. The container also has an inlet through which an inert gas, such as nitrogen, can be introduced into the container. The inert gas then circulates into the container around the leak area and thereby prevents any ambient air, such as oxygen, from entering the tube during a critical temperature ramp up moment that must take place to carry out the subsequent oxidation.
While the boat handling area is purged of oxygen and the tube is preheated, any oxygen which enters through the inadvertent leak areas in the absence of an oxide reducing gas, such as hydrogen, will, unfortunately, oxidize the refractory metal. When oxidation is carried out, both hydrogen and oxygen should be present in the form of steam. In the critical moments before oxidation, either the inert gas, hydrogen, or a combination of inert gas and hydrogen, are presentxe2x80x94not oxygen. Preferably, the inert gas and hydrogen is maintained during temperature ramp up and ramp down, with more inert gas being present than hydrogen. The inert gas and hydrogen is hereinafter referred to as a xe2x80x9cforming gas.xe2x80x9d After temperature ramp up and before ramp down, the inert gas is replaced with more hydrogen and oxygen, to form steam. The steam is alternatively referred to as an xe2x80x9coxidizing gas.xe2x80x9d The steam not only oxidizes the polysilicon sidewall surface, but also partially oxidizes the refractory metal sidewall surface. However, the hydrogen within the steam ambient will reduce whatever oxidation occurs on the refractory metal sidewall surface so that essentially little if any oxidation growth occurs on the refractory metal sidewall.
Instead of using a relatively cool tube that may be of the low-pressure variety, the present tube is preheated to enhance throughput and is purposely an atmospheric pressure tube or furnace. An atmospheric pressure oxidation furnace utilizes a torch to preheat the forming gas introduced into the tube. The forming gas is placed into the tube at atmospheric pressure, with any hydrogen present in the tube occurring after the tube is sealed to prevent a combustible condition.
According to one embodiment, an atmospheric pressure furnace is provided. The furnace includes a tube and a container. The container can be secured against the external surface of the tube so that a portion of the container forms an opening which abuts with the external surface and surrounds an opening, crack, aperture, or leak into the tube. Thus, the opening is alternatively referred to as a leak area. The container essentially covers the leak area and confines an inert gas received by the container to the leak area: This essentially prevents large amounts of ambient air containing oxygen from entering the leak area (relative to oxygen entering the leak area absent the nitrogen-filled containers) during a critical pre- or post-oxidation moment.
The opening or leak is one that is purposely not present by design. However, the opening or leak into the tube might occur if a seal or hinge, for example, on the inlet line, outlet line, or tube door were to leak. In addition to the container being usable to cover leaks within the tube, the container can also be used to cover leaks within a heating chamber or torch external to and separate from the tube. That is, in an atmospheric pressure furnace, a torch is needed to preheat gas introduced into the tube. Like the tube, the heating chamber can also develop inadvertent leaks. The container either completely or partially surrounds the entire heating chamber.
According to another embodiment, an apparatus is provided. The apparatus comprises the container having an inlet and an opening. The inlet serves to receive an inert gas into the container and the opening can be placed against an external region of a vessel, such as the tube or heating chamber (torch). The opening of the container is therefore one that can be configured to secure around a leak area of the vessel.
According to yet another embodiment, a method is provideand The method includes pushing wafers through a doorway into a heated tube and then closing the doorway. The temperature of the tube can then be increased in the presence of a forming gas, such gas being a majority (much greater than 60%) nitrogen and a minority (much less than 40%) hydrogen. Thereafter, the nitrogen can be removed and hydrogen flow increased, along with oxygen, to form steam while inert gas is maintained across select portions of the exterior surface of the tube that may or may not contain a leak. Preferably, however, the inert gas is thereby present should a leak form.